On the nature of X-ray-bright, optically normal galaxies

Recent X-ray surveys by Chandra and XMM-Newton have revealed a population of X-ray-bright, optically normal galaxies (XBONGs) at moderate redshifts. We propose that many XBONGs are powered by an inner radiatively inefficient accretion flow (RIAF) plus an outer radiatively efficient thin accretion disk. The absence of optical/UV activity in XBONGs is explained by the truncation of the thin disk near the black hole, while the relatively strong X-ray emission is explained as inverse Compton emission from the hot RIAF. As an example, we show that the spectra of two XBONGs can be fitted fairly well with such a model. By comparing these two sources to other accreting black holes, we argue that XBONGs are intermediate in their characteristics between distant, luminous active galactic nuclei and nearby, low-luminosity nuclei.